Identifiers begin with a letter (a letter is every letter in Unicode UTF-8 or _) and followed by 0 or more letters or Unicode digits, like: X56, group1, _x23, i, өԑ12.

The _ itself is a special identifier, called the blank identifier. It can be used in declarations or variable assignments like any other identifier (and any type can be assigned to it), but its value is discarded, so it cannot be used anymore in the code that follows.